1. Field of the Invention
The present invention relates generally to an electric power steering apparatus which provides power assist of an electric motor directly to a steering system so as to reduce necessary steering power to be applied by the driver. More particularly, it relates to such an electric power steering apparatus which is capable of detecting an operation failure of a current detecting unit provided to detect a current flowing through the electric motor.
2. Description of the Related Art
In general, the electric power steering apparatus includes an electric motor incorporated in a steering system and a control unit or controller which controls operation of the electric motor to eventually reduce a manual steering effort or force required by the driver.
FIG. 17 of the accompanying drawings shows in block diagram the general construction of a conventional controller 200A. The controller 200A includes a target current determining unit 210, a PWM (pulse width modulation) signal generating section or generator 220, and a motor drive unit 230. The target current determining unit 210 determines a target assist torque on the basis of a steering torque signal Tp and generates a target current signal IT required for an electric motor 100 to exert the target assist torque to a steering system. The PWM signal 220 generates, on the basis of the target current signal IT, a PWM signal as a drive control signal 220a which is used for controlling operation of the electric motor 100.
The motor drive unit 230 has a gate drive circuit 240 and an electric motor drive circuit 250 including four power FETs (field-effect transistors) connected together to form a bridge H-network. The gate drive circuit 240 drives, on the basis of the drive control signal (PWM signal) 220a, the gate of each of the FETs to turn on or conduct the individual FETs. Thus, on the basis of the steering torque signal Tp corresponding to a steering torque detected by a steering torque detecting unit 120, electric power supplied from a battery power source BAT to the electric motor 100 is altered into a pulse-like shape in an effort to control output power (steering assist torque) of the electric motor 100.
FIG. 18 shows in block diagram the construction of another conventional controller 200B. In the controller 200B, a motor current IM actually supplied to an electric motor 100 is detected so that a feedback control of the electric motor 100 is performed on the basis of the motor current IM, so as to improve the control characteristics of the electric motor 100. The controller 200B of this motor current feedback control type differs from the controller 200A shown in FIG. 17 in that it further includes a current detecting unit 260 for detecting a current flowing through the electric motor 100 so as to generate a motor current signal IM, an offset calculating unit 270 for determining an offset between the motor current signal IM and a target current signal IT, and a motor operation control unit 280 for generating, on the basis of the offset signal 270a generated from the offset calculating unit 270, a drive control signal 220a to drive the electric motor 100.
The motor operation control unit 280 includes a current feedback (F/B) control section 290 and a PWM signal generator 220. The current F/B control section 290 performs control actions, such as proportional, integral, derivative and so forth. for the offset signal to generate a drive current signal 290a for controlling a current to be supplied to the electric motor 100 in such a manner as to tend the offset toward zero. The PWM signal generator 220 generates, on the basis of the drive current signal 290a, a PWM signal for the drive control of the electric motor 100 and outputs the generated PWM signal as the above-mentioned drive control signal 220a.
The controller 200B of the motor current feedback control type shown in FIG. 18 is able to improve the control accuracy and response because operation of the electric motor 100 is controlled in such a manner that the offset between the target motor current IT and the actual motor current IM tends to zero.
However, when the current detecting unit 260 causes an operation failure which, for example, tends to fix the detected current at a particular value (including zero), the controller 200B of this type cannot perform the normal feedback control operation any more, failing to supply a desired steering assist torque to the steering system.
FIGS. 19A and 19B are graphs illustrative of an example of the operation failure in which the output (detected current) IM from the current detecting unit 260 is fixed at a constant value. FIG. 19A exemplifies the time-current characteristic of the target current IT and the detected current IM, and FIG. 19B shows the time-current characteristic of the drive control signal 220a generated from the motor operation control unit 280 on the basis of the offset 270a between the target current IT and the detected current IM shown in FIG. 19a. In the case where the output from the current detecting unit 260 is fixed at a current value IM.alpha., the motor operation control unit 280 generates, on the basis of an offset between a target current IT and the detected current IM.alpha., a drive control signal 220a tending to vary a current to be supplied to the electric motor 100 in such a manner as to tend the offset closer toward zero. In this instance, however, since the detected current value is fixed and does not vary any more, the drive control signal 220a generated from the motor operation control unit 280 has a larger level in the direction to cancel out the offset.
Japanese Patent Laid-open Publication No. HEI 8-91239 discloses a controller for an electric power steering apparatus, which can detect a failure in a motor current detecting unit. The disclosed controller is constructed to check the motor current detecting unit for a possible failure immediately after an ignition key is turned on. To this end, the controller applies a voltage V across an electric motor for a time T which is much less than a mechanical time constant Tm of the motor and which is much greater than an electric time constant Te of the motor (Te&lt;&lt;T&lt;&lt;Tm). While applying the voltage to the electric motor, the controller determines whether or not the motor current detecting unit is at fault, through a comparison made between an estimated motor current is which is estimated from a motor terminal voltage V and a motor terminal resistance R, and a motor current i appearing in a condition in which an angular velocity .omega. of the motor is approximately zero, i.e., the motor is substantially not rotating.
The controller disclosed in Japanese Patent Laid-open Publication No. HEI 8-91239 is so constructed as to check the motor current detecting unit (motor current detecting unit) against a failure immediately after the ignition key is turned on, and so this controller cannot detect any operation failure in the motor current detecting unit occurring during movement of the motor vehicle. In order to determine the estimated motor current is based on the motor terminal voltage V and the motor terminal resistance R, detection of a battery voltage is inevitable. As a consequence, the controller further requires a battery voltage detector and so on. Additionally, the motor terminal resistance R needs to be registered in advance. Accordingly, when the motor is replaced with a different type of motor, re-registration of the motor terminal resistance R is needed.